Simulating shock propagation in defective metals

In this work an overview of ongoing molecular dynamics simulations of shocks in metals is presented. Results for a perfect crystal are summarized, including how plasticity nucleates, the Hugoniot for different crystalline directions, and the formation of nano-twins. The role of different defect types in the Hugoniot is discussed. As an example, it is found that vacancies do not affect the Hugoniot, and that spherical voids affect the plastic threshold but they do not affect the slope of the Hugoniot for the void sizes studied (1-5 nm). The threshold pressure for collapse of a given void size is smaller than that obtained from simple quasi-static considerations. Shock propagation in nano-crystalline materials shows extra heating at the grain boundaries, leading to a reduced melting pressure. If a prismatic loop of certain orientation is introduced in a perfect crystal, it can act as a set of Frank-Read dislocation sources under shock compression. The activation of such a loop was studied as a function of pressure, together with the evolution of the induced 'dislocation density.' Finally, 'long' (100 ps) shock simulations for a Cu sample with a length close to one micron and including several dislocation sources are presented in order to study the evolution of plastic deformation after considerable plastic strain has taken place.